1. Field of the Invention
The present invention relates to techniques for forming contacts to electrodes or conductive interconnects consisting only or chiefly of aluminum, and the active layer of TFTs to make conductive interconnects.
2. Description of the Related Art
In recent years, techniques for fabricating thin-film transistors (TFTs) on cheap glass substrates have evolved rapidly, because there is an increasing demand for active matrix liquid crystal displays.
An active matrix liquid crystal display has millions of pixels arranged in rows and columns. TFTs are arranged at these pixels. Electric charge going into and out of each electrode at the pixels is controlled by the switching action of the TFTs.
An integrated circuit comprising a substrate on which both a display pixel portion and a driver circuit portion are formed has enjoyed general acceptance. Circuit TFTs for driving the pixel TFTs are incorporated in the peripheral driver circuit. The display pixel portion consists of a liquid crystal along with the pixel electrodes.
As mentioned above, an integrated circuit for an active matrix liquid crystal display is composed of as many as millions of pixel TFTs and hundreds of circuit TFTs. Therefore, the production yield is inevitably low.
For example, if one pixel TFT fails to operate, then pixel electrodes connected with this defective TFT do not operate as display elements. This gives rise to a so-called point defect. For example, in the case of a normally black liquid crystal display, when white is produced on the display device, a point defect appears as a black point, which is deeply harmful to the appearance.
Also, if any circuit TFT fails to operate, all the pixel TFTs applied with a drive voltage from this faulty TFT fail to act as switching elements. This results in a so-called line defect and is a fatal hindrance to the liquid crystal display.
Accordingly, in an active matrix liquid crystal display, millions of TFTs must operate normally and stably over a long term. However, the present situation is that it is difficult to exhaustively eliminate point defects and line defects. One of the causes is defective contact.
The defective contact is a defective operation when an interconnect electrode is electrically poorly connected with a TFT at a contact site. Especially, in the case of a planar TFT, defective contact presents serious problems, because an interconnect electrode is electrically connected with a TFT through a thin contact hole.
Defective contact is a main cause of early stage deterioration of semiconductor device characteristics. Especially, where large currents flow or the device is operated at high temperatures, the deterioration is promoted. Therefore, it is reasonably said that the reliability of contacts determines the reliability of the semiconductor device.
Generally, in the case of pixel display regions of an active matrix liquid crystal display, source electrodes are brought directly out of the pixel display regions and so there exist only contacts for connection with the semiconductor layer of the TFT.
In the case of a peripheral driver circuit, there are tens of thousands to millions of contacts. Especially, because there exist gate electrode contacts, and because the temperature is elevated by large-current operation, the contacts must have higher reliability than the pixel display regions.
The causes of defective contacts are classified into three major categories.
The first category is that a conductive film forming interconnect electrodes is not in ohmic contact with a semiconductor film forming the source/drain regions of TFTs.
This is caused by formation of an insulating coating such as a metal oxide at the junction plane. Also, the states in the vicinities of the semiconductor film surface (doping concentration, defect level density, level of cleanliness, and so on) greatly affect the performance of the contacts.
The second category is that the conductive film forming the interconnect electrodes has poor coverage and thus the metal lines break within contact holes. In this case, it is necessary to improve the situation by the method of forming the interconnect electrodes or by film growth conditions.
The third category is that an interconnect electrode breaks due to the cross-sectional shape of the contact hole. The cross-sectional shape of the contact hole depends heavily on the conditions under which the insulators (SiN, SiO.sub.2, etc.) covered with the contact portions are etched.
Especially, recessing caused by overetching and blowholes severely deteriorate the coverage and thus pose great problems. The manner in which a gate electrode is recessed is described by referring to FIG. 14.
FIG. 14 is an enlarged view of a contact hole portion that permits a gate electrode of a planar TFT to make contact with conductive interconnects.
In FIG. 14(A), a gate electrode 11 is made from a metallization material that can be anodized. In this embodiment, this metallization material consists chiefly of aluminum (Al). For simplicity, a gate insulator film, a semiconductor layer, and so on existing under the gate electrode 11 are omitted in the figure.
An anodic oxide film 12 is formed by anodizing the gate electrode 11 within an electrolyte. This anodic oxide film consists mainly of A1.sub.2 O.sub.3.
This anodic oxide film 12 is very dense and firm and protects the gate electrode 11 from heat produced during thermal treatment. Hence, formation of hillocks and whiskers is suppressed.
An interlayer dielectric film 13 is formed on the gate electrode 11. A silicide film such as silicon oxide film, silicon nitride film, or silicon oxynitride film can be used as the interlayer dielectric film 13.
Then, the interlayer dielectric film 13 is etched by wet etching or dry etching to form a contact hole 14. For this purpose, the interlayer dielectric film 13 consisting of a silicide film must be first etched, followed by etching of the anodic oxide film 12.
However, the anodic oxide film 12 is very dense and firm and so it takes a considerable time to perform the etching. Therefore, the etching progresses considerably laterally, thus forming a recessed portion 15 as shown in FIG. 14(B).
Under this condition, an interconnect electrode 16 is deposited. This state is shown in FIG. 14(C). As can be seen from this figure, the interconnect electrode 16 cannot cover the recessed portion. This can cause breaks in metal lines.
If overetching is prolonged at the end of the etching of the anodic oxide film, the etching of the gate electrode 11 progresses slowly. This may result in blowholes. Also in this case, there arises the problem that metal lines break.